Battery

ABSTRACT

Provided is a battery capable of improving chemical stability of an electrolyte and battery characteristics such as discharge capacity, charge-discharge cycle characteristics and so on. The battery comprises a spirally wound electrode body ( 20 ) including a strip-shaped cathode ( 21 ) and a strip-shaped anode ( 22 ) spirally wound with a separator ( 23 ) in between. During charge, lithium metal is precipitated on the anode ( 22 ), so the capacity of the anode ( 22 ) includes a capacity component by insertion and extraction of lithium and a capacity component by precipitation and dissolution of the lithium metal, and is represented by the sum of them. The separator ( 23 ) is impregnated with an electrolyte solution formed through dissolving a lithium salt in a solvent. Vinyl ethylene carbonate, divinyl ethylene carbonate or the like is added to the electrolyte solution, thereby the chemical stability can be improved.

TECHNICAL FIELD

The present invention relates to a battery comprising a cathode, ananode, and an electrolyte, and more specifically a battery in which thecapacity of the anode includes a capacity component by insertion andextraction of light metal and a capacity component by precipitation anddissolution of the light metal, and is represented by the sum of them.

BACKGROUND ART

In recent years, reduction in size and weight of portable electricdevices typified by cellular phones, PDAs (personal digital assistants)or laptop computers has been vigorously pursued, and as part of thereduction, an improvement in energy density of batteries, specificallysecondary batteries as power sources for the devices has been stronglyrequired.

As secondary batteries which can obtain a high energy density, forexample, a lithium-ion secondary battery using a material capable ofinserting and extracting lithium (Li) such as a carbon material or thelike for the anode is cited. The lithium-ion secondary battery isdesigned so that lithium inserted into an anode material is always in anion state, so the energy density is highly dependent on the number oflithium ions capable of being inserted into the anode material.Therefore, in the lithium-ion secondary battery, it is expected thatwhen the amount of insertion of lithium is increased, the energy densitycan be further improved. However, the amount of insertion of graphite,which is considered at present to be a material capable of the mosteffectively inserting and extracting lithium ions is theoreticallylimited to 372 mAh per gram on an electricity amount basis, and recentlythe amount of insertion of graphite has been approaching the limit byactive development.

Further, as the secondary battery capable of obtaining a high energydensity, a lithium secondary battery using lithium metal for an anode,and using only precipitation and dissolution reactions of lithium metalfor an anode reaction is also cited. In the lithium secondary battery, atheoretical electrochemical equivalent of the lithium metal is as largeas 2054 mAh/cm³, which is 2.5 times larger than that of graphite used inthe lithium-ion secondary battery, so it is expected that the lithiumsecondary battery can obtain a much higher energy density than thelithium-ion secondary battery. A large number of researchers have beenconducting research and development aimed at putting the lithiumsecondary battery to practical use (for example, Lithium Batteriesedited by Jean-Paul Gabano, Academic Press, 1983, London, N.Y.).

However, the lithium secondary battery has a problem that when acharge-discharge cycle is repeated, a large decline in its dischargecapacity occurs, so it is difficult to put the lithium secondary batteryto practical use. The decline in the capacity occurs because the lithiumsecondary battery uses a precipitation-dissolution reaction of thelithium metal in the anode. In accordance with charge and discharge, thevolume of the anode largely increases or decreases by the amount of thecapacity corresponding to lithium ions transferred between the cathodeand the anode, so the volume of the anode is largely changed, thereby itis difficult for a dissolution reaction and a recrystallization reactionof a lithium metal crystal to reversibly proceed. Further, the higherenergy density the lithium secondary battery achieves, the more largelythe volume of the anode is changed, and the more pronouncedly thecapacity declines.

Therefore, the inventors of the invention have developed a novelsecondary battery in which the capacity of the anode includes a capacitycomponent by insertion and extraction of lithium and a capacitycomponent by precipitation and dissolution of lithium, and isrepresented by the sum of them (refer to International Publication No.WO 01/22519). In the secondary battery, a carbon material capable ofinserting and extracting lithium is used for the anode, and lithium isprecipitated on a surface of the carbon material during charge. Thesecondary battery holds promise of improving charge-discharge cyclecharacteristics while achieving a higher energy density.

However, in order to put the secondary battery to practical use, it isrequired to achieve a further improvement in the characteristics andhigher stability. For this purpose, research and development of not onlyelectrode materials but also electrolytes are absolutely necessary. Morespecifically, when a side reaction between an electrolyte and anelectrode occurs, and a side reaction product is deposited on a surfaceof the electrode, an internal resistance of the battery increases,thereby the charge-discharge cycle characteristics pronouncedly decline.Further, when lithium is consumed at this time, it may result in adecline in capacity. In short, chemical stability of the electrolyte isa very important issue.

In view of the foregoing, it is an object of the invention to provide abattery capable of improving chemical stability of an electrolyte, andbattery characteristics such as discharge capacity, charge-dischargecycle characteristics and so on.

DISCLOSURE OF THE INVENTION

A battery according to the invention comprises a cathode, an anode andan electrolyte, wherein the capacity of the anode includes a capacitycomponent by insertion and extraction of light metal and a capacitycomponent by precipitation and dissolution of the light metal, and isrepresented by the sum of them, and the electrolyte includes at leastone kind selected from the group consisting of a compound shown inChemical Formula 1 and a compound shown in Chemical Formula 2.

In the battery according to the invention, in an insertion-extractionreaction of the light metal, reduction and decomposition of a solventcan be inhibited, and in a precipitation-dissolution reaction of thelight metal, a reaction between precipitated light metal and the solventcan be prevented. Therefore, chemical stability of the electrolyte ishigher, so a higher discharge capacity can be obtained, and cyclecharacteristics or the like can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a secondary battery according to anembodiment of the invention; and

FIG. 2 is an enlarged sectional view of a part of a spirally electrodebody in the secondary battery shown in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the invention will be described below in moredetail below referring to the accompanying drawings.

FIG. 1 shows a sectional view of a secondary battery according to anembodiment of the invention. The secondary battery is a so-calledcylindrical type, and comprises a spirally wound electrode body 20including a strip-shaped cathode 21 and a strip-shaped anode 22 spirallywound with a separator 23 in between in a substantially hollowcylindrical-shaped battery can 11. The battery can 11 is made of, forexample, nickel (Ni)-plated iron. An end portion of the battery can 11is closed, and the other end portion thereof is opened. In the batterycan 11, a pair of insulating plates 12 and 13 are disposed so that thespirally wound electrode body 20 is sandwiched therebetween in adirection perpendicular to a spirally wound peripheral surface.

In the opened end portion of the battery can 11, a battery cover 14 and,a safety valve mechanism 15 and a positive temperature coefficientdevice (PTC device) 16 disposed inside the battery cover 14 are mountedthrough caulking by a gasket 17, and the interior of the battery can 11is sealed. The battery cover 14 is made of, for example, the samematerial as that of the battery can 11. The safety valve mechanism 15 iselectrically connected to the battery cover 14 through the PTC device16, and when internal pressure in the battery increases to higher than acertain extent due to an internal short circuit or external applicationof heat, a disk plate 15 a is flipped so as to disconnect the electricalconnection between the battery cover 14 and the spirally wound electrodebody 20. When a temperature rises, the PTC device 16 limits a current byan increased resistance, thereby resulting in preventing abnormal heatgeneration by a large current. The PTC device 16 is made of, forexample, barium titanate semiconductor ceramic. The gasket 17 is madeof, for example, an insulating material, and its surface is coated withasphalt.

The spirally wound electrode body 20 is wound around, for example, acenter pin 24. A cathode lead 25 made of aluminum (Al) or the like isconnected to the cathode 21 of the spirally wound electrode body 20, andan anode lead 26 made of nickel or the like is connected to the anode22. The cathode lead 25 is welded to the safety valve mechanism 15 so asto be electrically connected to the battery cover 14, and the anode lead26 is welded to the battery can 11 so as to be electrically connected tothe battery can 11.

FIG. 2 shows an enlarged view of a part of the spirally wound electrodebody 20 shown in FIG. 1. The cathode 21 has, for example, a structure inwhich a cathode mixed layer 21 b is disposed on both sides of a cathodecurrent collector 21 a having a pair of surfaces facing each other. Inaddition, the cathode mixed layer 21 b may be disposed on only one sideof the cathode current collector 21 a, although it is not shown. Thecathode current collector 21 a is made of, for example, metal foil suchas aluminum foil, nickel foil, stainless foil or the like with athickness of approximately from 5 μm to 50 μm. The cathode mixed layer21 b has, for example, a thickness of 80 μm to 250 μm, and includes acathode material capable of inserting and extracting lithium which islight metal. Further, when the cathode mixed layer 21 b is disposed onboth sides of the cathode current collector 21 a, the thickness of thecathode mixed layer 21 b means the total thickness thereof.

As the cathode material capable of inserting and extracting lithium, forexample, a lithium-containing compound such as a lithium oxide, alithium sulfide, an intercalation compound including lithium or the likeis adequate, and a mixture including two or more kinds selected fromthem may be used. More specifically, in order to achieve a higher energydensity, a lithium complex oxide or an intercalation compound includinglithium represented by a general formula Li_(x)MO₂ is preferable. In theformula, as M, one or more kinds of transition metals, more specificallyat least one kind selected from the group consisting of cobalt (Co),nickel, manganese (Mn), iron (Fe), aluminum, vanadium (V) and titanium(Ti) is preferable. The value of x depends upon a charge-discharge stateof the battery, and is generally within a range of 0.05≦x≦1.10. Inaddition, LiMn₂O₄ having a spinel crystal structure, LiFePO₄ having anolivine crystal structure, or the like is preferable, because a higherenergy density can be obtained.

Further, such a cathode material is prepared through the followingsteps. For example, after a carbonate, a nitrate, an oxide or ahydroxide including lithium, and a carbonate, a nitrate, an oxide or ahydroxide including a transition metal are mixed so as to have a desiredcomposition, and the mixture is pulverized, the pulverized mixture isfired at a temperature ranging from 600° C. to 1000° C. in an oxygenatmosphere, thereby the cathode material is prepared.

The cathode mixed layer 21 b includes, for example, an electronicconductor, and may further include a binder, if necessary. As theelectronic conductor, for example, a carbon material such as graphite,carbon black, ketjen black or the like is cited, and one kind or amixture of two or more kinds selected from them is used. In addition tothe carbon material, any electrically conductive material such as ametal material, a conductive high molecular weight material or the likemay be used. As the binder, for example, synthetic rubber such asstyrene butadiene rubber, fluorine rubber, ethylene propylene dienerubber or the like, or a high molecular weight material such aspolyvinylidene fluoride or the like is cited, and one kind or a mixtureincluding two or more kinds selected from them is used. For example, asshown in FIG. 1, when the cathode 21 and the anode 22 are spirallywound, the styrene butadiene rubber, the fluorine rubber or the likehaving high elasticity is preferably used as the binder.

The anode 22 has, for example, a structure in which an anode mixed layer22 b is disposed on both sides of an anode current collector 22 a havinga pair of surfaces facing each other. The anode mixed layer 22 b may bedisposed on only one side of the anode current collector 22 a, althoughit is not shown. The anode current collector 22 a is made of, forexample, metal foil having excellent electrochemical stability, electricconductivity and mechanical strength such as copper foil, nickel foil,stainless foil or the like. More specifically, the copper foil is themost preferable because the copper foil has high electric conductivity.The anode current collector 22 a preferably has a thickness of, forexample, approximately 6 μm to 40 μm. When the thickness of the anodecurrent collector 22 a is thinner than 6 μm, the mechanical strengthdeclines, so the anode current collector 22 a is easily broken during amanufacturing process, thereby production efficiency declines. On theother hand, when it is thicker than 40 μm, a volume ratio of the anodecurrent collector 22 a in the battery becomes larger than necessary, soit is difficult to increase the energy density.

The anode mixed layer 22 b includes one kind or two or more kindsselected from anode materials capable of inserting and extractinglithium which is light metal, and may further include, for example, thesame binder as that included in the cathode mixed layer 21 b, ifnecessary. The anode mixed layer 22 b has a thickness of, for example,80 μm to 250 μm. When the anode mixed layer 22 b is disposed on bothsides of the anode current collector 22 a, the thickness of the anodemixed layer 22 b means the total thickness thereof.

In this description, insertion and extraction of light metal mean thatlight metal ions are electrochemically inserted and extracted withoutlosing their ionicity. It includes not only the case where insertedlithium metal exists in a perfect ion state but also the case where theinserted lithium metal exists in an imperfect ion state. As these cases,for example, insertion by electrochemical intercalation of light metalions into graphite is cited. Further, insertion of the light metal intoan alloy including an intermetallic compound, or insertion of the lightmetal by forming an alloy can be cited.

As the anode material capable of inserting and extracting lithium, forexample, a carbon material such as graphite, non-graphitizable carbon,graphitizing carbon or the like is cited. These carbon materials arepreferable, because a change in the crystalline structure which occursduring charge and discharge is extremely small, so a highercharge-discharge capacity and superior charge-discharge cyclecharacteristics can be obtained. Further, graphite is more preferable,because its electrochemical equivalent is large, and a higher energydensity can be obtained.

For example, graphite with a true density of 2.10 g/cm³ or over ispreferable, and graphite with a true density of 2.18 g/cm³ or over ismore preferable. In order to obtain such a true density, a c-axiscrystalline thickness of a (002) plane is required to be 14.0 nm orover. Moreover, the spacing of (002) planes is preferably less than0.340 nm, and more preferably within a range from 0.335 nm to 0.337 nm.

The graphite may be natural graphite or artificial graphite. Theartificial graphite can be obtained through the following steps, forexample. An organic material is carbonized, and high-temperature heattreatment is carried out on the carbonized organic material, then theorganic material is pulverized and classified so as to obtain theartificial graphite. The high-temperature treatment is carried out inthe following steps. For example, the organic material is carbonized at300° C. to 700° C. in an airflow of an inert gas such as nitrogen (N₂)or the like, if necessary, and then the temperature rises to 900° C. to1500° C. at a rate of 1° C. to 100° C. per minute, and the temperatureis kept for 0 to 30 hours to calcine the organic material, then theorganic material is heated to 2000° C. or over, preferably 2500° C. orover, and the temperature is kept for an adequate time.

As the organic material as a starting material, coal or pitch can beused. As the pitch, for example, a material which can be obtained bydistillation (vacuum distillation, atmospheric distillation or steamdistillation), thermal polycondensation, extraction, and chemicalpolycondensation of tars which can be obtained by thermally crackingcoal tar, ethylene bottom oil, crude oil or the like at hightemperature, asphalt or the like, a material produced during destructivedistillation of wood, a polyvinyl chloride resin, polyvinyl acetate,polyvinyl butyrate, or a 3,5-dimethylphenol resin is cited. These coalsand pitches exist in a liquid state around at 400° C. at the highestduring carbonization, and by keeping the coal and pitches at thetemperature, aromatic rings are condensed and polycycled, so thearomatic rings are aligned in a stacking arrangement. After that, asolid carbon precursor, that is, semi-coke is formed at approximately500° C. or over (liquid-phase carbonization process).

Moreover, as the organic material, a condensed polycyclic hydrocarboncompound such as naphthalene, phenanthrene, anthracene, triphenylene,pyrene, perylene, pentaphene, pentacene or the like, a derivativethereof (for example, carboxylic acid of the above compound, carboxylicacid anhydride, carboxylic acid imide), or a mixture thereof can beused. Further, a condensed heterocyclic compound such as acenaphthylene,indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine,carbazole, acridine, phenazine, phenanthridine or the like, a derivativethereof, or a mixture thereof can be used.

In addition, pulverization may be carried out before or aftercarbonization and calcination, or during a rise in temperature beforegraphitization. In these cases, the material in powder form is heatedfor graphitization in the end. However, in order to obtain graphitepowder with a higher bulk density and a higher fracture strength, it ispreferable that after the material is molded, the molded material isheated, then the graphitized molded body is pulverized and classified.

For example, in order to form the graphitized molded body, after coke asa filler and binder pitch as a molding agent or a sintering agent aremixed and molded, a firing step in which the molded body is heated at alow temperature of 1000° C. or less and a step of impregnating the firedbody with the molten binder pitch are repeated several times, and thenthe body is heated at high temperature. The binder pitch with which thefired body is impregnated is carbonized by the above heat treatmentprocess so as to be graphitized. In this case, the filler (coke) and thebinder pitch are used as the materials, so they are graphitized as apolycrystal, and sulfur or nitrogen included in the materials isgenerated as a gas during the heat treatment, thereby minute pores areformed in a path of the gas. Therefore, there are some advantages thatinsertion and extraction of lithium proceed more easily by the pores,and industrial processing efficiency is higher. Further, as the materialof the molded body, a filler having moldability and sinterability may beused. In this case, the binder pitch is not required.

The non-graphitizable carbon having the spacing of the (002) planes of0.37 nm or over and a true density of less than 1.70 g/cm³, and notshowing an exothermic peak at 700° C. or over in a differential thermalanalysis (DTA) in air is preferable.

Such non-graphitizable carbon can be obtained, for example, throughheating the organic material around at 1200° C., and pulverizing andclassifying the material. Heat treatment is carried out through thefollowing steps. After, if necessary, the material is carbonized at 300°C. to 700° C. (solid phase carbonization process), a temperature risesto 900° C. to 1300° C. at a rate of 1° C. to 100° C. per minute, and thetemperature is kept for 0 to 30 hours. Pulverization may be carried outbefore or after carbonization or during a rise in temperature.

As the organic material as a starting material, for example, a polymeror a copolymer of furfuryl alcohol or furfural, or a furan resin whichis a copolymer including macromolecules thereof and any other resin canbe used. Moreover, a conjugated resin such as a phenolic resin, anacrylic resin, a vinyl halide resin, a polyimide resin, a polyamideimide resin, a polyamide resin, polyacetylene, polyparaphenylene or thelike, cellulose or a derivative thereof, coffee beans, bamboos,crustacea including chitosan, kinds of bio-cellulose using bacteria canbe used. Further, a compound in which a functional group includingoxygen (O) is introduced into petroleum pitch with, for example, a ratioH/C of the number of atoms between hydrogen (H) and carbon (C) of from0.6 to 0.8 (that is, an oxygen cross-linked compound) can be used.

The percentage of the oxygen content in the compound is preferably 3% orover, and more preferably 5% or over (refer to Japanese UnexaminedPatent Application Publication No. Hei 3-252053). The percentage of theoxygen content has an influence upon the crystalline structure of acarbon material, and when the percentage is the above value or over, thephysical properties of the non-graphitizable carbon can be improved,thereby the capacity of the anode 22 can be improved. Moreover, thepetroleum pitch can be obtained, for example, by distillation (vacuumdistillation, atmospheric distillation or steam distillation), thermalpolycondensation, extraction, and chemical polycondensation of tarsobtained through thermally cracking coal tar, ethylene bottom oil orcrude oil at high temperature, asphalt or the like. Further, as a methodof forming an oxygen cross-link, for example, a wet method of reacting asolution such as nitric acid, sulfuric acid, hypochlorous acid, amixture thereof or the like and petroleum pitch, a dry method ofreacting an oxidizing gas such as air, oxygen or the like and petroleumpitch, or a method of reacting a solid reagent such as sulfur, ammoniumnitrate, ammonium persulfate, ferric chloride or the like and petroleumpitch can be used.

In addition, the organic material as the starting material is notlimited to them, and any other organic material which can becomenon-graphitizable carbon through the solid-phase carbonization by anoxygen cross-linking process or the like may be used.

As the non-graphitizable carbon, in addition to the non-graphitizablecarbon formed of the above organic material as a starting material, acompound including phosphorus (P), oxygen and carbon as main componentswhich is disclosed in Japanese Unexamined Patent Application PublicationNo. Hei 3-137010 is preferable, because the above-described parametersof physical properties are exhibited.

As the anode material capable of inserting and extracting lithium, ametal element or a metalloid element capable of forming an alloy withlithium, or an alloy of the metal element or the metalloid element, or acompound of the metal element or the metalloid element is cited. Theyare preferable because a higher energy density can be obtained, and itis more preferable to use them with a carbon material, because a higherenergy density and superior cycle characteristics can be obtained. Inthe description, the alloy means not only an alloy including two or morekinds of metal elements but also an alloy including one or more kinds ofmetal elements and one or more kinds of metalloid elements. As thecomposition of the alloy, a solid solution, a eutectic (eutecticmixture), an intermetallic compound or the coexistence of two or morekinds selected from them is cited.

As such a metal element or a metalloid element, for example, tin (Sn),lead (Pb), aluminum, indium (In), silicon (Si), zinc (Zn), antimony(Sb), bismuth (Bi), cadmium (Cd), magnesium (Mg), boron (B), gallium(Ga), germanium (Ge), arsenic (As), silver (Ag), zirconium (Zr), yttrium(Y) or hafnium (Hf) is cited. As an alloy or a compound thereof, forexample, an alloy or a compound represented by a chemical formulaMa_(s)Mb_(t)Li_(u) or a chemical formula Ma_(p)Mc_(q)Md_(r) is cited. Inthese chemical formulas, Ma represents at least one kind selected frommetal elements and metalloid elements which can form an alloy or acompound with lithium, Mb represents at least one kind selected frommetal elements and metalloid elements except for lithium and Ma, Mcrepresents at least one kind selected from nonmetal elements, and Mdrepresents at least one kind selected from metal elements and metalloidelements except for Ma. Further, the values of s, t, u, p, q and r ares>0, t≧0, u≧0, p>0, q>0 and r≧0, respectively.

Among them, a metal element or a metalloid element selected from Group4B, or an alloy thereof or a compound thereof is preferable, and siliconor tin, or an alloy thereof or a compound thereof is more preferable.They may have a crystalline structure or an amorphous structure.

As specific examples of such an alloy or such a compound, LiAl, AlSb,CuMgSb, SiB₄, SiB₆, Mg₂Si, Mg₂Sn, Ni₂Si, TiSi₂, MoSi₂, CoSi₂, NiSi₂,CaSi₂, CrSi₂, Cu₅Si, FeSi₂, MnSi₂, NbSi₂, TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC,Si₃N₄, Si₂N₂O, SiO_(v) (0<v≦2), SnO_(w) (0<w≦2), SnSiO₃, LiSiO, LiSnOand so on are cited.

Moreover, as the anode material capable of inserting and extractinglithium, other metal compounds or high molecular weight materials arecited. As the metal compounds, an oxide such as iron oxide, rutheniumoxide, molybdenum oxide or the like, LiN₃, and so on are cited, and asthe high molecular weight materials, polyacetylene, polyaniline,polypyrrole and so on are cited.

Moreover, in the secondary battery, during charge, precipitation oflithium metal on the anode 22 begins at a point where an open circuitvoltage (that is, battery voltage) is lower than an overcharge voltage.In other words, in a state where the open circuit voltage is lower thanthe overcharge voltage, the lithium metal is precipitated on the anode22, so the capacity of the anode 22 includes a capacity component byinsertion and extraction of lithium and a capacity component byprecipitation and dissolution of the lithium metal, and is representedby the sum of them. Therefore, in the secondary battery, both of theanode material capable of inserting and extracting lithium and thelithium metal have a function as an anode active material, and the anodematerial capable of inserting and extracting lithium is a base materialwhen the lithium metal is precipitated.

The overcharge voltage means a open circuit voltage when the battery isovercharged, and indicates, for example, a voltage higher than the opencircuit voltage of a battery “fully charged” described in and defined by“Guideline for safety assessment of lithium secondary batteries” (SBAG1101) which is one of guidelines drawn up by Japan Storage Batteryindustries Incorporated (Battery Association of Japan). In other words,the overcharge voltage indicates a higher voltage than an open circuitvoltage after charge by using a charging method used when a nominalcapacity of each battery is determined, a standard charging method or arecommended charging method. More specifically, the secondary battery isfully charged, for example, at a open circuit voltage of 4.2 V, and thelithium metal is precipitated on a surface of the anode material capableof inserting and extracting lithium in a part of the range of the opencircuit voltage of from 0 V to 4.2 V.

Thereby, in the secondary battery, a higher energy density can beobtained, and cycle characteristics and high-speed chargecharacteristics can be improved, because of the following reason. Thesecondary battery is equivalent to a conventional lithium secondarybattery using lithium metal or a lithium alloy for the anode in a sensethat the lithium metal is precipitated on the anode. However, in thesecondary battery, the lithium metal is precipitated on the anodematerial capable of inserting and extracting lithium, thereby it isconsidered that the secondary battery has the following advantages.

Firstly, in the conventional lithium secondary battery, it is difficultto uniformly precipitate the lithium metal, which causes degradation incycle characteristics, however, the anode material capable of insertingand extracting lithium generally has a large surface area, so in thesecondary battery, the lithium metal can be uniformly precipitated.Secondly, in the conventional lithium secondary battery, a change involume according to precipitation and dissolution of the lithium metalis large, which also causes degradation in the cycle characteristics;however, in the secondary battery, the lithium metal is precipitated ingaps between particles of the anode material capable of inserting andextracting lithium, so a change in volume is small. Thirdly, in theconventional lithium secondary battery, the larger the amount ofprecipitation and dissolution of the lithium metal is, the bigger theabove problem becomes; however, in the secondary battery, insertion andextraction of lithium by the anode material capable of inserting andextracting lithium contributes to a charge-discharge capacity, so inspite of a large battery capacity, the amount of precipitation anddissolution of the lithium metal is small. Fourthly, when theconventional lithium secondary battery is quickly charged, the lithiummetal is more nonuniformly precipitated, so the cycle characteristicsare further degraded. However, in the secondary battery, in an initialcharge, lithium is inserted into the anode material capable of insertingand extracting lithium, so the secondary battery can be quickly charged.

In order to more effectively obtain these advantages, for example, it ispreferable that at the maximum voltage before the open circuit voltagebecomes an overcharge voltage, the maximum capacity of the lithium metalprecipitated on the anode 22 is from 0.05 times to 3.0 times larger thanthe charge capacity of the anode material capable of inserting andextracting lithium. When the amount of precipitation of the lithiummetal is too large, the same problem as the problem which occurs in theconventional lithium secondary battery arises, and when the amount istoo small, the charge-discharge capacity cannot be sufficientlyincreased. Moreover, for example, the discharge capacity of the anodematerial capable of inserting and extracting lithium is preferably 150mAh/g or over. The larger the ability to insert and extract lithium is,the smaller the amount of precipitation of the lithium metal relativelybecomes. In addition, the charge capacity of the anode material isdetermined by the quantity of electricity when the battery with theanode made of the anode material as an anode active material and thelithium metal as a counter electrode is charged by a constant-currentconstant-voltage method until reaching 0 V. For example, the dischargecapacity of the anode material is determined by the quantity ofelectricity when the battery is subsequently discharged in 10 hours ormore by a constant-current method until reaching 2.5 V.

The separator 23 is made of, for example, a porous film of a syntheticresin such as polytetrafluoroethylene, polypropylene, polyethylene orthe like, or a porous film of ceramic, and the separator 23 may have astructure in which two or more kinds of the porous films are laminated.Among them, a porous film made of polyolefin is preferably used, becauseby use of the porous film, a short circuit can be effectively prevented,and the safety of the battery can be improved by a shutdown effect. Morespecifically, polyethylene can obtain a shutdown effect within a rangeof from 100° C. to 160° C., and is superior in electrochemicalstability, so polyethylene is preferably used as the material of theseparator 23. Moreover, polypropylene is also preferably used, and anyother resin having chemical stability can be used by copolymerizing orblending with polyethylene or polypropylene.

The porous film made of polyolefin is obtained through the followingsteps, for example. After a molten polyolefin composite is kneaded witha molten low-volatile solvent in liquid form to form a solutionuniformly containing a high concentration of the polyolefin composite,the solution is extruded through a die, and is cooled to form a gel-formsheet, then the gel-form sheet is drawn to obtain the porous film.

As the low-volatile solvent, for example, a low-volatile aliphatic groupsuch as nonane, decane, decalin, p-xylene, undecane, liquid paraffin orthe like, or a cyclic hydrocarbon can be used. A composition ratio ofthe polyolefin composite and the low-volatile solvent is preferably 10wt % to 80 wt % of the polyolefin composite, and more preferably 15 wt %to 70 wt % of the polyolefin composite, when the total ratio of thepolyolefin composite and the low-volatile solvent is 100 wt %. When thecomposition ratio of the polyolefin composite is too small, duringformation, swelling or neck-in becomes large at the exit of the die, soit is difficult to form the sheet. On the other hand, when thecomposition ratio of the polyolefin composite is too large, it isdifficult to prepare a uniform solution.

When the solution containing a high concentration of the polyolefincomposite is extruded through the die, in the case of a sheet die, a gappreferably has, for example, 0.1 mm to 5 mm. Moreover, it is preferablethat an extrusion temperature is within a range of from 140° C. to 250°C., and an extrusion speed is within a range of from 2 cm/minute to 30cm/minute.

The solution is cooled to at least a gelling temperature or less. As acooling method, a method of directly making the solution contact withcooling air, cooling water, or any other cooling medium, a method ofmaking the solution contact with a roll cooled by a cooling medium orthe like can be used. Moreover, the solution containing a highconcentration of the polyolefin composite which is extruded from the diemay be pulled before or during cooling at a pulling ratio of from 1 to10, preferably from 1 to 5. It is not preferable to pull the solution ata too large pulling ratio, because neck-in becomes large, and a rupturetends to occur during drawing.

It is preferable that, for example, the gel-form sheet is heated, andthen is biaxially drawn through a tenter process, a roll process, arolling process, or a combination thereof. At this time, eithersimultaneous drawing in all direction or sequential drawing may be used,but simultaneous secondary drawing is preferable. The drawingtemperature is preferably equivalent to or lower than a temperature of10° C. higher than the melting point of the polyolefin composite, andmore preferably a crystal dispersion temperature or over and less thanthe melting point. A too high drawing temperature is not preferable,because effective molecular chain orientation by drawing cannot beachieved due to melting of the resin, and when the drawing temperatureis too low, softening of the resin is insufficient, thereby a rupture ofthe gel-form sheet tends to occur during drawing, so the gel-form sheetcannot be drawn at a high enlargement ratio.

After drawing the gel-form sheet, the drawn film is preferably cleanedwith a volatile solvent to remove the remaining low-volatile solvent.After cleaning, the drawn film is dried by heating or air blasting tovolatilize the cleaning solvent. As the cleaning solvent, for example,an easily volatile material, that is, a hydrocarbon such as pentane,hexane, heptane or the like, a chlorinated hydrocarbon such as methylenechloride, carbon tetrachloride or the like, a fluorocarbon such astrifluoroethane or the like, ether such as diethyl ether, dioxane or thelike is used. The cleaning solvent is selected depending upon the usedlow-volatile solvent, and one kind selected from the cleaning solventsor a mixture thereof is used. A method of immersing in the volatilesolvent to extract, a method of sprinkling the volatile solvent, or acombination thereof can be used for cleaning. Cleaning is performeduntil the remaining low-volatile solvent in the drawn film becomes lessthan 1 part by mass relative to 100 parts by mass of the polyolefincomposite.

The separator 23 is impregnated with an electrolyte solution which is aliquid electrolyte. The electrolyte solution includes a liquid solvent,for example, a nonaqueous solvent such as an organic solvent or thelike, and a lithium salt which is an electrolyte salt dissolved in thenonaqueous solvent. The liquid nonaqueous solvent is made of, forexample, a nonaqueous compound with an intrinsic viscosity of 10.0 mPa.sor less at 25° C. The nonaqueous solvent with an intrinsic viscosity of10.0 mPa.s or less in a state that the electrolyte salt is dissolvedtherein may be used, and in the case where a plurality of kinds ofnonaqueous compounds are mixed to form a solvent, the solvent may havean intrinsic viscosity of 10.0 mPa.s or less in a state that thecompounds are mixed.

As such a nonaqueous solvent, various nonaqueous solvents conventionallyused can be used. More specifically, cyclic carbonate such as propylenecarbonate, ethylene carbonate or the like, chain ester such as diethylcarbonate, dimethyl carbonate, ethyl methyl carbonate or the like, ethersuch as γ-butyrolactone, sulfolane, 2-methyl tetrahydrofuran,dimethoxyethane or the like is cited. More specifically, in terms ofoxidation stability, it is preferable to use the nonaqueous solventmixed with carbonate.

As the lithium salt, for example, LiAsF₆, LiPF₆, LiBF₄, LiClO₄,LiB(C₆H₅)₄, LiCH₃SO₃, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂,LiN(C₄F₉SO₂)(CF₃SO₂), LiC(CF₃SO₂)₃, LiAlCl₄, LiSiF₆, LiCl or LiBr iscited, and one kind or a mixture including two or more kinds selectedfrom them may be used.

Among them, LiPF₆ is preferable, because a higher conductivity can beobtained, and oxidation stability is superior, and LiBF₄ is preferable,because thermal stability and oxidation stability are superior.Moreover, LiCF₃SO₃ is preferable, because thermal stability is higher,and LiClO₄ is preferable, because a higher conductivity can be obtained.Further, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂ and LiC(CF₃SO₂)₃ are preferable,because relatively high conductivity can be obtained, and thermalstability is high. Further, a mixture including at least two kindsselected from them is preferably used, because a combination of theseeffects can be obtained. More specifically, a mixture including at leastone kind selected from the group consisting of lithium salts having amolecular structure shown in Chemical Formula 3 such as LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, LiC(CF₃SO₂)₃ and so on and one or more kinds of otherlithium salts except for the lithium salts having the molecularstructure shown in Chemical Formula 3 is more preferably used, becausehigher conductivity can be obtained, and chemical stability of theelectrolyte solution can be improved. As the other lithium salt,specifically LiPF₆ is preferable.

The content (concentration) of the lithium salt in the solvent ispreferably within a range of 0.5 mol/kg to 3.0 mol/kg, becausesufficient battery characteristics may not be obtained out of the range,because of a pronounced decline in ionic conductivity.

The electrolyte solution also includes at least one kind selected fromthe group consisting of a compound shown in Chemical Formula 4 and acompound shown in Chemical Formula 5 as an additive. Thereby, in thesecondary battery, reduction and decomposition of the solvent can beinhibited in an insertion-extraction reaction of lithium, and a reactionbetween precipitated lithium metal and the solvent can be prevented in aprecipitation-dissolution reaction of lithium. In other words, chemicalstability of the electrolyte solution is improved, so a higher dischargecapacity can be obtained, and cycle characteristics can be improved. Inaddition, the above compounds may function as a solvent, however, in thedescription, attention is given to the above function, so the compoundis described as the additive. At least a part of the added compound maycontribute to the above-described reaction, and the compound notcontributing to the reaction may function as a solvent.

As the compound shown in Chemical Formula 4, for example, vinyl ethylenecarbonate shown in Chemical Formula 6, vinyl ethylene trithiocarbonateshown in Chemical Formula 7, or 1,3-butadiene ethylene carbonate shownin Chemical Formula 8 is cited. As the compound shown in ChemicalFormula 5, for example, divinyl ethylene carbonate shown in ChemicalFormula 9 is cited. Among them, vinyl ethylene carbonate shown inChemical Formula 6 or divinyl ethylene carbonate shown in ChemicalFormula 9 is preferably included, because higher effects can beobtained.

In the case where two or more kinds of compounds are included, the totalcontent of these compounds is preferably within a range of 0.005 wt % to15 wt % relative to the total of the solvent and the electrolyte salt.When the content is less than 0.005 wt %, no sufficient effect can beobtained, and when the content is larger than 15 wt %, degradation inthe battery during storage may occur.

Moreover, instead of the electrolyte solution, a gel electrolyte inwhich a high molecular weight compound holds an electrolyte solution maybe used. Any gel electrolyte having an ionic conductivity of 1 mS/cm orover at room temperature may be used, and the composition of the gelelectrolyte and the structure of the high molecular weight compound arenot specifically limited. The electrolyte solution (that is, the liquidsolvent, the electrolyte salt and the additive) is as described above.As the high molecular weight compound, for example, polyacrylonitrile,polyvinylidene fluoride, a copolymer of polyvinylidene fluoride andpolyhexafluoropropylene, polytetrafluoroethylene,polyhexafluoropropylene, polyethylene oxide, polypropylene oxide,polyphosphazene, polysiloxane, polyvinyl acetate, polyvinyl alcohol,polymethylmethacrylate, polyacrylic acid, polymethacrylic acid,styrene-butadiene rubber, nitrile-butadiene rubber, polystyrene orpolycarbonate is cited. Specifically, in terms of electrochemicalstability, a high molecular weight compound having the structure ofpolyacrylonitrile, polyvinylidene fluoride, polyhexafluoropropylene orpolyethylene oxide is preferably used. An amount of the high molecularweight compound added to the electrolyte solution varies depending uponcompatibility between them, however, in general, an amount of the highmolecular weight compound equivalent to 5 wt % to 50 wt % of theelectrolyte solution is preferably added.

Moreover, the content of the compound shown in Chemical Formula 4 or 5and the content of the lithium salt are the same as in the case of theelectrolyte solution. Herein, the solvent widely means not only a liquidsolvent but also a material capable of dissociating the electrolyte saltand having ionic conductivity. Therefore, when a high molecular weightcompound with ionic conductivity is used as the high molecular weightcompound, the high molecular weight compound is also considered to be asolvent.

The secondary battery can be manufactured through the following steps,for example.

At first, for example, a cathode material capable of inserting andextracting lithium, an electronic conductor, and a binder are mixed toprepare a cathode mixture, and the cathode mixture is dispersed in asolvent such as N-methyl-2-pyrrolidone or the like to produce cathodemixture slurry in paste form. After the cathode mixture slurry isapplied to the cathode current collector 21 a, and the solvent is dried,the cathode mixed layer 21 b is formed through compression molding by aroller press or the like so as to form the cathode 21.

Next, for example, an anode material capable of inserting and extractinglithium and a binder are mixed to prepare an anode mixture, then theanode mixture is dispersed in a solvent such as N-methyl-2-pyrrolidoneor the like to produce anode mixture slurry in paste form. After theanode mixture slurry is applied to the anode current collector 22 a, andthe solvent is dried, the anode mixed layer 22 b is formed throughcompression molding by a roller press or the like so as to form theanode 22.

Then, the cathode lead 25 is attached to the cathode current collector21 a by welding or the like, and the anode lead 26 is attached to theanode current collector 22 a by welding or the like. After that, forexample, a laminate including the cathode 21 and anode 22 with theseparator 23 in between is spirally wound, and an end portion of thecathode lead 25 is welded to the safety valve mechanism 15, and an endportion of the anode lead 26 is welded to the battery can 11. Then, thespirally wound laminate including the cathode 21 and the anode 22sandwiched between a pair of insulating plates 12 and 13 is contained inthe battery can 11. After the spirally wound laminate including thecathode 21 and the anode 22 is contained in the battery can 11, theelectrolyte is injected into the battery can 11, and the separator 23 isimpregnated with the electrolyte. After that, the battery cover 14, thesafety valve mechanism 15 and the PTC device 16 are fixed in an openedend portion of the battery can 11 through caulking by the gasket 17.Thereby, the secondary battery shown in FIG. 1 is formed.

The secondary battery works as follows.

In the secondary battery, when charge is carried out, lithium ions areextracted from the cathode mixed layer 21 b, and are inserted into theanode material capable of inserting and extracting lithium included inthe anode mixed layer 22 b through the electrolyte with which theseparator 23 is impregnated. When the charge further continues, in astate where the open circuit voltage is lower than the overchargevoltage, the charge capacity exceeds the charge capacity of the anodematerial capable of inserting and extracting lithium, and then lithiummetal begins to be precipitated on the surface of the anode materialcapable of inserting and extracting lithium. After that, until thecharge is completed, precipitation of lithium metal on the anode 22continues. Thereby, for example, when graphite is used as the anodematerial capable of inserting and extracting lithium, the color of thesurface of the anode mixed layer 22 b changes from black to gold, andthen to silver.

Next, when discharge is carried out, at first, the lithium metalprecipitated on the anode 22 is eluted as ions, and is inserted into thecathode mixed layer 21 b through the electrolyte with which theseparator 23 is impregnated. When the discharge further continues,lithium ions inserted into the anode material capable of inserting andextracting lithium in the anode mixed layer 22 b are extracted, and areinserted into the cathode mixed layer 21 b through the electrolyte.Therefore, in the secondary battery, the characteristics of theconventional lithium secondary battery and the lithium-ion secondarybattery, that is, a higher energy density and superior charge-dischargecycle characteristics can be obtained.

More specifically in the embodiment, at least one selected from thecompounds shown in Chemical Formulas 4 and 5 is included, so whenlithium is inserted into the anode 22, in a radical active site,unsaturated alkyl groups R1, R2 and R3 in Chemical Formula 4 or 5 react,then these compounds are polymerized with each other by ring-openingpolymerization, or are absorbed by the anode material capable ofinserting and extracting lithium or polymerized with the anode materialby ring-opening polymerization, so a film is formed on a surface of theanode 22. Thereby, reduction and decomposition of the solvent in theradical active site of the anode 22 can be inhibited. Moroever, thecompound formed by the above reaction has a cyclic carbonate structure.For example, compared to a compound formed by ring-openingpolymerization of vinylene carbonate, the degree of freedom of an oxogroup which functions as a lithium ion conducting medium is high, so thefilm is considered to be a dense film with lithium ion conductivity.Therefore, it is considered that the precipitation of lithium metal iscarried out under the film, and in a precipitation-dissolution reactionof lithium, a reaction between precipitated lithium metal and thesolvent can be prevented by the film. Further, the film stably remainson the surface of the anode 22 even after the dissolution of lithium, sothe above function is sustained in charge and discharge thereafter.

Thus, in the embodiment, at least one selected from the compounds shownin Chemical Formulas 4 and 5 is included, so when lithium is insertedinto the anode 22, the unsaturated alkyl group R1, R2 and R3 in ChemicalFormula 4 or 5 react in the radical active site so that the film can beformed on the surface of the anode 22, thereby reduction anddecomposition of the solvent in the radical active site of the anode 22can be inhibited. Moreover, in a precipitation-dissolution reaction oflithium, precipitation of lithium metal can be carried out under thefilm, so a reaction between the precipitated lithium metal and thesolvent can be prevented. Therefore, the chemical stability of theelectrolyte can be improved, and battery characteristics such asdischarge capacity, charge-discharge cycle characteristics and so on canbe improved.

More specifically, when the content of the above compound is within arange of 0.005 wt % to 15 wt % relative to the total of the solvent andthe electrolyte salt, higher effects can be obtained.

Next, specific examples of the invention will be described in moredetail below referring to FIGS. 1 and 2.

EXAMPLES 1 THROUGH 4

Batteries in which the area density ratio of the cathode 21 and theanode 22 was adjusted, and the capacity of the anode 22 included acapacity component by insertion and extraction of lithium and a capacitycomponent by precipitation and dissolution of the lithium, and wasrepresented by the sum of them were formed.

At first, lithium carbonate (Li₂CO₃) and cobalt carbonate (CoCO₃) weremixed at a ratio (molar ratio) of Li₂CO₃:CoCO₃=0.5:1, and the mixturewas fired in air at 900° C. for 5 hours to obtain lithium cobalt complexoxide (LiCoO₂) as the cathode material. Next, 91 parts by weight oflithium cobalt complex oxide, 6 parts by weight of graphite as anelectronic conductor and 3 parts by weight of polyvinylidene fluoride asa binder were mixed to prepare a cathode mixture. Then, the cathodemixture was dispersed in N-methyl-2-pyrrolidone as a solvent to formcathode mixture slurry. After the cathode mixture slurry was uniformlyapplied to both sides of the cathode current collector 21 a made ofstrip-shaped aluminum foil with a thickness of 20 μm, and was dried.Then, the cathode mixed layer 21 b was formed through compressionmolding by a roller press so as to form the cathode 21. After that, thecathode lead 25 made of aluminum was attached to an end of the cathodecurrent collector 21 a.

Moreover, artificial graphite powder was prepared as an anode material,and 90 parts by weight of the artificial graphite powder and 10 parts byweight of polyvinylidene fluoride as a binder were mixed to prepare ananode mixture. Next, the anode mixture was dispersed inN-methyl-2-pyrrolidone as a solvent to form anode mixture slurry. Afterthe anode mixture slurry was uniformly applied to both sides of theanode current collector 22 a made of strip-shaped copper foil with athickness of 10 μm, and was dried. Then, the anode mixed layer 22 b wasformed through compression molding by a roller press so as to form theanode 22. Next, the anode lead 26 made of nickel was attached to an endof the anode current collector 22 a.

After the cathode 21 and the anode 22 were formed, the separator 23 madeof a porous polypropylene film with a thickness of 25 μm was prepared.Then, a laminate including the anode 22, the separator 23, the cathode21 and the separator 23 in this order was spirally wound several timesto form the spirally wound electrode body 20.

After the spirally wound electrode body 20 was formed, the spirallywound electrode body 20 was sandwiched between a pair of insulatingplates 12 and 13, and the anode lead 26 was welded to the battery can11, and the cathode lead 25 was welded to the safety valve mechanism 15.Then, the spirally wound electrode body 20 was contained in the batterycan 11 made of nickel-plated iron. After that, the electrolyte solutionwas injected into the battery can 11 by a decompression method. As theelectrolyte solution, a mixed solvent of 50 vol % of ethylene carbonateand 50 vol % of diethyl carbonate with LiPF₆ as the electrolyte saltdissolved therein at a ratio of 1 mol/dm³ to which vinyl ethylenecarbonate shown in Chemical Formula 6 added thereto was used. At thattime, the content of vinyl ethylene carbonate relative to the total ofthe solvent and the electrolyte salt varied in Examples 1 through 4 asshown in Table 1.

After the electrolyte solution was injected into the battery can 11, thebattery cover 14 was caulked into the battery can 11 by the gasket 17 ofwhich a surface was coated with asphalt so as to obtain the cylindricalsecondary batteries with a diameter of 14 mm and a height of 65 mm ofExamples 1 through 4 were formed.

As Comparative Example 1 relative to Examples, a secondary battery wasformed as in the case of Examples, except that vinyl ethylene carbonatewas not added to the electrolyte solution. Moreover, as ComparativeExamples 2 and 3 relative to Examples, lithium-ion secondary batterieswere formed as in the case of Examples, except that the area densityratio of the cathode and the anode was adjusted, and the capacity of theanode was represented by insertion and extraction of lithium. At thattime, in Comparative Example 2, a vinyl ethylene carbonate content of 2wt % relative to the solvent was added to the electrolyte solution, andin Comparative Example 3, vinyl ethylene carbonate was not added to theelectrolyte solution.

A charge-discharge test was carried out on the secondary batteries ofExamples 1 through 4 and Comparative Examples 1 throuth 3 to determine adischarge capacity in the first cycle, that is, an initial dischargecapacity, and a discharge capacity in the 100th cycle. At that time,charge was carried out at a constant current of 600 mA until a batteryvoltage reached 4.2 V, then the charge was continued at a constantvoltage of 4.2 V until a current reached 1 mA. Discharge was carried outat a constant current of 400 mA until the battery voltage reached 3.0 V.When charge and discharge were carried out under the conditions, thebatteries were in a full charge condition and a full dischargecondition. The obtained results are shown in Table 1. In Table 1, theinitial discharge capacity of each of Examples 1 through 4 was arelative value when the initial discharge capacity of ComparativeExample 1 was 100, and the discharge capacity of each of Examples 1through 4 in the 100th cycle was a relative value when the dischargecapacity of Comparative Example 1 in the 100th cycle was 100. Moreover,the initial discharge capacity of Comparative Example 2 was a relativevalue when the initial discharge capacity of Comparative Example 3 was100, and the discharge capacity of Comparative Example 2 in the 100thcycle was a relative value when the discharge capacity of ComparativeExample 3 in the 100th cycle was 100.

Moreover, after the first cycle of charge and discharge was carried outon the secondary batteries of Examples 1 through 4 and ComparativeExamples 1 through 3 under the above-described conditions, the batterieswere fully charged again, then the fully charged batteries weredisassembled to check whether the lithium metal was precipitated on theanode mixed layer 22 b by visual inspections and ⁷Li nuclear magneticresonance spectroscopy. Further, the second cycle of charge anddischarge was carried out under the above-described conditions to fullydischarge the batteries, then the batteries were disassembled to checkwhether the lithium metal was precipitated on the anode mixed layer 22 bin a like manner.

As a result, in the secondary batteries of Examples 1 through 4 andComparative Example 1, in a full charge condition, precipitation oflithium metal on the anode mixed layer 22 b was observed, and in a fulldischarge condition, no precipitation of lithium metal was observed. Inother words, it was confirmed that the capacity of the anode 22 includeda capacity component by precipitation and dissolution of lithium metaland a capacity component by insertion and extraction of lithium metal,and was represented by the sum of them. In Table 1, Y denotes thepresence of precipitated lithium metal.

On the other hand, in the secondary batteries of Comparative Examples 2and 3, in a full charge condition and a full discharge condition, noprecipitation of lithium metal was observed, and only the presence oflithium ions was observed. Moreover, a peak attributed to the observedlithium ions was extremely small. In other words, it was confirmed thatthe capacity of the anode was represented by insertion and extraction oflithium. In Table 1, N denotes the absence of precipitated lithiummetal.

It was obvious from Table 1 that the batteries of Examples 1 through 4in which vinyl ethylene carbonate was added to the electrolyte solutioncould obtain the initial discharge capacity and the discharge capacityin the 100th cycle equivalent to or higher than the battery ofComparative Example 1 in which no vinyl ethylene carbonate was added tothe electrolyte solution, and specifically the discharge capacity in the100th cycle could be improved more than that in Comparative Example 1.On the other hand, in the lithium-ion secondary batteries of ComparativeExamples 2 and 3, the lithium-ion secondary battery of ComparativeExample 2 in which vinyl ethylene carbonate was added to the electrolytesolution could obtain a slightly higher initial discharge capacity and aslightly higher discharge capacity in the 100th cycle than thelithium-ion secondary battery of Comparative Example 3 in which no vinylethylene carbonate was added to the electrolyte solution, however,compared to Example 2 in which the same content of vinyl ethylenecarbonate was added to the electrolyte solution, vinyl ethylenecarbonate in Comparative Example 2 produced a little effect. In otherwords, it was found out that the secondary battery in which the capacityof the anode 22 included the capacity component by insertion andextraction of light metal and the capacity component by precipitationand dissolution of light metal, and was represented by the sum of them,when vinyl ethylene carbonate was included in the electrolyte solution,the discharge capacity and the charge-discharge cycle characteristicscould be improved.

Moreover, it was found out from the results of Examples 1 through 4 thatin accordance with an increase in the vinyl ethylene carbonate content,there was a tendency of the initial discharge capacity and the dischargecapacity in the 100th cycle to increase, and decrease after reaching themaximum value. In other words, it was found out that when the vinylethylene carbonate content in the electrolyte solution was within arange of 0.005 wt % to 15 wt % relative to the total of the solvent andthe electrolyte salt, higher effects could be obtained.

EXAMPLES 5 THROUGH 7

Secondary batteries were formed as in the case of Example 2, except thatinstead of vinyl ethylene carbonate, vinyl ethylene trithiocarbonateshown in Chemical Formula 7, 1,3-butadiene ethylene carbonate shown inChemical Formula 8, or divinyl ethylene carbonate shown in ChemicalFormula 9 was added to the electrolyte solution. The charge-dischargetest was carried out on Examples 5 through 7 as in the case of Example 2to determine the initial discharge capacity and the discharge capacityin the 100th cycle, and to check whether the lithium metal wasprecipitated in a full charge condition and in a full dischargecondition. The results are shown in Table 2 together with the results ofExample 2 and Comparative Example 1. In Table 2, the initial dischargecapacity was a relative value when the initial capacity of ComparativeExample 1 was 100, and the discharge capacity in the 100th cycle was arelative value when the discharge capacity of Comparative Example 1 inthe 100th cycle was 100.

It was obvious from Table 2 that as in the case of Example 2, thesecondary batteries of Examples 5 through 7 could obtain the initialdischarge capacity and the discharge capacity in the 100th cycle higherthan those in Comparative Example 1. In other words, it was found outthat when the compound shown in Chemical Formula 4 or 5 was included inthe electrolyte solution, the discharge capacity and thecharge-discharge cycle characteristics could be improved.

In the above examples, the description is given referring to specificexamples of the compound shown in Chemical Formula 4 or Chemical Formula5; however, it is considered that the above-described effects resultfrom the molecular structure shown in Chemical Formula 4 or ChemicalFormula 5. Therefore, the same effects can be obtained by using anyother compound shown in Chemical Formula 4 or Chemical Formula 5.Moreover, in the above examples, the case where the electrolyte solutionis used is described, although the same effects can be obtained by usinga gel electrolyte.

The present invention is described referring to the embodiment and theexamples, but the invention is not limited to the above embodiment andthe examples, and is variously modified. For example, in the embodimentand the examples, the case where lithium is used as light metal isdescribed; however, the invention can be applied to the case where anyother alkali metal such as sodium (Na), potassium (K) or the like,alkaline-earth metal such as magnesium, calcium (Ca) or the like, anyother light metal such as aluminum or the like, lithium, or an alloythereof is used, thereby the same effects can be obtained. In this case,the anode material capable of inserting and extracting light metal, thecathode material, the nonaqueous solvent, the electrolyte salt or thelike is selected depending upon the light metal. However, lithium or analloy including lithium is preferably used as the light metal, becausevoltage compatibility with lithium-ion secondary batteries which arepractically used at present is high. Further, when the alloy includinglithium is used as the light metal, a material capable of forming analloy with lithium may be present in the electrolyte or the anode so asto form an alloy during precipitation.

Moreover, in the above embodiments and the examples, the case where theelectrolyte solution or the gel electrolyte which is a kind of solidelectrolyte is used is described, but any other electrolyte may be used.As the electrode, for example, an organic solid electrolyte in which anelectrolyte salt is dispersed in a high molecular weight compound havingionic conductivity, an inorganic solid electrolyte made ofion-conductive ceramic, ion-conductive glass, ionic crystal or the like,a mixture of the inorganic solid electrolyte and an electrolytesolution, a mixture of the inorganic solid electrolyte and the gelelectrolyte, or a mixture of the inorganic solid electrolyte and theorganic solid electrolyte is cited.

Further, in the above embodiment and the examples, the cylindrical typesecondary battery with a spirally wound structure is described; however,the invention is applicable to an elliptic type or a polygonal typesecondary battery with a spirally wound structure, or a secondarybattery with a structure in which the cathode and anode are folded orlaminated in a like manner. In addition, the invention is applicable toa secondary battery with a coin shape, a button shape, a prismaticshape, a large size or the like. Further, the invention is applicable tonot only the secondary batteries but also primary batteries.

As described above, in the battery according to the invention, theelectrolyte includes at least one kind selected from the compounds shownin Chemical Formula 1 and Chemical Formula 2, so when light metal isinserted into the anode, the unsaturated alkyl group R1, R2 and R3 reactin the radical active site, so a film can be formed on the surface ofthe anode. Thereby, reduction and decomposition of the solvent in theradical active site of the anode can be inhibited. Moreover, in theprecipitation-dissolution reaction of light metal, the precipitation ofthe light metal can be carried out under the film, so a reaction betweenthe precipitated light metal and the solvent can be prevented.Therefore, the chemical stability of the electrolyte can be improved,and the battery characteristics such as the discharge capacity, thecharge-discharge cycle characteristics and so on can be improved.

More specifically, in the battery according to an aspect of theinvention, the content of the compound shown in Chemical Formula 1 orChemical Formula 2 is within a range of 0.005 wt % to 15 wt % relativeto the total of the solvent and the electrolyte salt, so higher effectscan be obtained.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

(where each of U, V and W represents one kind of Group 6B element, and R1 represents an unsaturated alkyl group.)

(where each of X, Y and Z represents one kind of Group 6B element, andeach of R 2 and R 3 represents an unsaturated alkyl group.)(CaFbSOc)d   (CHEMICAL FORMULA 3)(where each of a, b, c and d represents any number except for 0.)

(where each of U, V and W represents one kind of Group 6B element, and R1 represents an unsaturated alkyl group.)

(where each of X, Y and Z represents one kind of Group 6B element, andeach of R 2 and R 3 represents an unsaturated alkyl group.)

TABLE 1 INITIAL 100TH VINYL DIS- CYCLE PRE- ETHYLENE CHARGE DISCHARGECIPI- CARBONATE CAPACITY CAPACITY TATION (WT %) (mAh) (mAh) OF Li EXAM-0.005 101 101 Y PLE 1 EXAM- 2 111 115 Y PLE 2 EXAM- 10 105 105 Y PLE 3EXAM- 15 99 102 Y PLE 4 COMPARA- 0 100 100 Y TIVE EX- AMPLE 1 COMPARA- 2102 103 N TIVE EX- AMPLE 2 COMPARA- 0 100 100 N TIVE EX- AMPLE 3

TABLE 2 INITIAL 100TH DIS- CYCLE CHARGE DISCHARGE PRECIPI- KIND OFCAPACITY CAPACITY TATION ADDITIVE (mAh) (mAh) OF Li EXAM- VINYL 111 115Y PLE 2 ETHYLENE CARBONATE EXAM- VINYL 106 110 Y PLE 5 ETHYLENE TRITHIO-CARBONATE EXAM- 1,3-BUTADIENE 104 108 Y PLE 6 ETHYLENE CARBONATE EXAM-DIVINYL 108 113 Y PLE 7 ETHYLENE CARBONATE COM- — 100 100 Y PARA- TIVEEXAM- PLE 1

1. A battery, comprising: a cathode; an anode; and an electrolyte,wherein the capacity of the anode includes a capacity component byinsertion and extraction of light metal and a capacity component byprecipitation and dissolution of the light metal, and is represented bythe sum of them, and the electrolyte include at least one kind selectedfrom the group consisting of a compound shown in Chemical Formula 10 anda compound shown in Chemical Formula
 11. 2. A battery according to claim1, wherein the electrolyte further includes a solvent and an electrolytesalt, and the total content of the compound shown in Chemical Formula 10and the compound shown in Chemical Formula 11 is within a range of 0.005wt % to 15 wt % relative to the total of the solvent and the electrolytesalt.
 3. A battery according to claim 1, wherein the electrolyteincludes vinyl ethylene carbonate.
 4. A battery according to claim 1,wherein the electrolyte includes divinyl ethylene carbonate.
 5. Abattery according to claim 1, wherein the electrolyte includes a mixtureof at least one kind of lithium salt selected from the group consistingof LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂ and LiC(CF₃SO₂)₃ and one or more kinds ofother lithium salts.
 6. A battery according to claim 1, wherein theanode includes an anode material capable of inserting and extractinglight metal.
 7. A battery according to claim 6, wherein the anodeincludes a carbon material.
 8. A battery according to claim 7, whereinthe anode includes at least one kind selected from the group consistingof graphite, graphitizing carbon and non-graphitizable carbon.
 9. Abattery according to claim 8, wherein the anode includes graphite.
 10. Abattery according to claim 6, wherein the anode includes at least onekind selected from the group consisting of single substances, alloys andcompounds of metal elements capable of forming an alloy with the lightmetal and metalloid elements capable of forming an alloy with the lightmetal.
 11. A battery according to claim 10, wherein the anode includesat least one kind selected from the group consisting of singlesubstances, alloys and compounds of tin (Sn), lead (Pb), aluminum (Al),indium (In), silicon (Si), zinc (Zn), antimony (Sb), bismuth (Bi),cadmium (Cd), magnesium (Mg), boron (B), gallium (Ga), germanium (Ge),arsenic (As), silver (Ag), zirconium (Zr), yttrium (Y) and hafnium (Hf).12. A battery according to claim 1, wherein the electrolyte includes ahigh molecular weight compound.

(where each of U, V and W represents one kind of Group 6B element, and R1 represents an unsaturated alkyl group.)

(where each of X, Y and Z represents one kind of Group 6B element, andeach of R 2 and R 3 represents an unsaturated alkyl group.)